Insulating films on metals, including alloys of metals and metallic compounds, are useful for a variety of applications, including insulation of devices and construction of devices such as microprocessor components. One particularly useful application of insulating films is the fabrication of tunnel junctions. These junctions are prepared by sandwiching a thin layer of a material which is normally an insulator between two conducting metals. The conductors may both be non-superconducting metals; they may both be superconducting metals; or one metal may be superconducting while the other metal is non-superconducting. A superconductor is any material which exhibits superconductivity at any temperature, preferably at a temperature above 0.01K. Tunnel junctions between two non-superconducting metals can be used, for example, to make capacitors or resistors having conductance properties which are not dependent on temperature. Tunnel junctions between two superconductors, known as Josephson junctions, can be used, for example, to make a transistor which can be used in a superconducting computer.
The term “metal” as used herein refers to substances made of a single metal such as gold, silver, copper, platinum, or lead; to substances made from an alloy of two or more metals; and to substances made from a metallic compound, optionally containing non-metallic elements. Examples of metal alloys include niobium-germanium alloys (NbGe) and niobium-titanium alloys (NbTi). Examples of metallic compounds include ceramic superconductors and oxide ceramic superconductors, such as YBa2Cu3O7 (YBCO).
Superconductors, especially oxide ceramic superconductors, are complex materials that are difficult to prepare in a tunnel junction. It has been extremely difficult to coat the surface of oxide superconductors with a thin insulating film without disordering the surface of the superconductor and/or yielding the insulating film with defects, such as pinholes. Oxide superconductors are also known to be environmentally sensitive and tend to degrade in atmospheres which are not inert.
In order for tunneling to occur through an insulating layer, the layer should be as thin as possible. Tunneling current decreases in an exponential fashion with the thickness of the substance through which the tunneling occurs. Thus, a film which is thick will not allow a tunneling current which is large enough to be useful. Preferably, an insulating tunnel barrier has a thickness of 30 angstroms (Å) or less.
One class of useful insulating films is the metal oxide family of general formula MOx (where “M” is a metal and x is from 0.01 to 4), which can be prepared through a variety of techniques. For example, chemical vapor deposition (CVD) of a metal oxide can be accomplished by treatment of the surface with a vaporized metal or a vaporized metal alkoxide of the general formula M(OR)y, where y is from 1 to 8, and R is an alkyl group. “Alkyl” refers to a substituted or unsubstituted, straight, branched or cyclic hydrocarbon chain containing from 1 to 20 carbon atoms. The chemisorbed layer formed is then treated with an activating agent such as an oxidizing agent or water, or by exposure to heat or light. (Toda et al. Langmuir, 11, 3281 (1995)) In another example, a metal alkoxide can be adsorbed onto the surface from a solution. The surface layer can then be heated to break down the metal alkoxide, followed by treatment of the surface with water to form the MOx film.
Lead oxide (PbOx) films can be formed on superconducting materials by the thermal evaporation of lead onto the surface of the superconductor, typically YBa2Cu3O7 (YBCO). It is believed that the lead oxide film is formed utilizing oxygen atoms present in the superconductor. This technique disorders the surface of the oxide layer, and this disorder adversely affects the reproducibility and stability of the tunneling process. Bismuth oxide (BiOx) films can be formed in a similar manner, but with similar disordering of the surface.
Sol-gel processes may also used to produce oxide films on surfaces. These pocesses involve deposition of a metal alkoxide onto the surface from a solution, hydrolysis of the chemisorbed metal alkoxide, and drying of the oxide film produced. See, for example, Ichinose et al. Chemistry Letters, 831-832 (1996); Ichinose et al. Chemistry of Materials, 9, 1296-1298 (1997); and Japanese patent application JP 09241008 A2. Although this process also produces a disordered surface structure, it does enable the preparation of oxide films incorporating a variety of metal species. Metal oxides of particular interest include zirconium oxides and oxides of similar metals, such as hafnium and titanium. These oxides have been especially difficult to use in preparing reproducible, ultrathin films. The resistances of the oxide films formed by conventional techniques is too high for the films to be useful in sensitive devices such as a Josephson junction.
It is thus desirable to provide oxide films on metals which are extremely thin, yet without defects or holes. Preferably, these oxide films can be formed in a reproducible fashion. High-quality, ultrathin oxide films would likely be useful as tunnel junctions between superconductors and metals.